With the constant development of the SMPS (Switched Mode Power Supply) technology, the SMPS has become very common in a portable device (such as a mobile phone, a notebook computer, a tablet computer, a laptop, etc.). For an AMOLED (Active Matrix/Organic Light Emitting Diode) panel power supply, not only large current capacity and accurate output voltage are required, low output voltage ripple is also an important parameter. In order to provide a comfortable lighting for human eyes, how to design a power supply with a stable output and a low ripple for AMOLED panel is a very important issue for the portable device.
To satisfy the above needs, a constant-frequency current-mode-controlled power converter is usually used to implement an SMPS with a stable output and a low voltage ripple. Compared with an SMPS of other structure (e.g., a voltage-mode-controlled converter, a constant-on variable-frequency converter, a constant-off variable-frequency converter), the constant operation frequency of the constant-frequency current-mode-controlled power converter makes it easier to reduce the frequency spectrum interference to other blocks in the system.
The constant-frequency current-mode-controlled power converter comprises a boost converter, a buck converter, and a buck-boost converter. For example, FIG. 1 shows a principle diagram of a constant-frequency current-mode-controlled boost converter according to the prior art. FIG. 2 shows a waveform associated with the constant-frequency current-mode-controlled boost converter. With reference to FIGS. 1 and 2, suppose the boost converter has reached a steady state and the clock and sawtooth wave generator 101 generates a clock pulse at a certain time, then the clock pulse sets the output Q (i.e., the drive signal) of a RS flip-flop 102 to high, and the driver circuit 103 receives the signal outputted by the RS flip-flop 102 and turns on an NMOS transistor Mn through an internal logic and signal Driver_N, such that two PMOS transistors Mp1 and Mp2 are cut off by signals Driver_P; at this point, the input voltage Vin charges the inductor L, thereby increasing the current IL in the inductor L; when the current IL of the inductor L reaches a value set by the output VC of the error amplifier 104, a PWM comparator 105 generates a pulse, such that the RS flip-flop 102 is toggled, and the NMOS transistor Mn is cut off, the two PMOS transistors Mp1 and Mp2 are turned on; then the inductor L starts discharging. When the boost converter is operated in a steady state, it repeats the above procedure. The operating principles of the buck converter and the buck-boost converter are similar to the boost converter, which will not be detailed here.
Line transient response is also an important parameter for describing the output characteristics of the power converter. For example, for a boost converter, it is crucial to improve the line transient response in the boost converter. In order to improve the line transient response, a method that can be immediately contemplated is increasing the bandwidth. With the increase of the bandwidth, the response time of the entire power converter will be reduced, such that the line transient response of the power converter is improved. However, due to existence of a right plane zero, its bandwidth is limited into a very small region, which cannot be increased unlimitedly. Therefore, this method cannot improve the line transient response well. Particularly when the power converter operates at a minimum input voltage and a maximum output voltage, the bandwidth is very small, so the line transient response will get worse.
FIG. 3 is a circuit diagram of a constant-frequency current-mode-controlled boost converter having a slope compensation according to the prior art. With reference to FIG. 3, if a compensation capacitor CC in FIG. 3 is decreased, the bandwidth of the boost converter will be increased, but it is hard for the boost converter to become steady when the boost converter is in a maximum duty cycle and heavy load condition. Although decrease of the loop gain of the boost converter can also increase the bandwidth, it will decrease the accuracy of the output voltage. Therefore, it is hard to balance among the bandwidth, loop gain, accuracy, and stability in the boost converter.
However, reducing the slope compensation might be an easier way to improve line transient response.
Refer to FIG. 3 again, in which a relationship between VSUM and VC is presented, as expressed by the following equation:VSUM=VSENSE+VSLOPE=VC  (1)It may be seen from equation (1) that if the slope compensation VSLOPE is decreased, VSUM will be decreased.
FIG. 4 shows variations of the output VC of the error amplifier and the output VOUT of the boost converter when the input voltage VIN varies in the boost converter of FIG. 3. With reference to FIG. 4, when the input voltage VIN varies from VI to VI−ΔV, the boost converter will change from state 1 to state 2 , and the duty cycle changes from
            D      ⁢                          ⁢      1        =                  VO        -        VI            VO        to            D      ⁢                          ⁢      2        =                            VO          -          VI          +                      Δ            ⁢                                                  ⁢            V                          VO            .      
FIG. 5 shows variation of VSUM with time for different slope compensations in the boost converter of FIG. 3. With reference to FIG. 5, the slope of VSUM1 is mc1, and the slope of VSUM2 is mc2. It is seen that VSUM1>VSUM2, VSLOPE1>VSLOPE2, mc1>mc2. When the duty cycle varies from D1 to D2, the following equations may be derived:ΔVSUM1=ΔVC1=mc1*(D2−D1)*T  (2)ΔVSUM2=ΔVC2=mc2*(D2−D1)*T  (3)ΔVC1>ΔVC2  (4)
It is seen that from state 1 to state 2, the variation of the VC value is larger when the slope compensation is larger. Moreover, it is known that the larger the variation ΔVC of the output VC of the error amplifier is, the greater is the variation on the output voltage VOUT. Therefore, when a smaller slope compensation is used, the variation on the VOUT will become even smaller, which means that the line transient response will improve when a smaller slope compensation is used.
FIG. 6 shows the simulation results of line transient response at a 1× slope and a 3× slope compensation. With reference to FIG. 6, it is seen that the larger VSUM is, the greater the variation on the output voltage VOUT is, i.e., VSUM1>VSUM2, dV1>dV2.
However, the above method of improving line transient response through decreasing the slope compensation has many problems. FIG. 7 shows variation of inductor current of the boost converter of FIG. 3 for different slope compensations. With reference to FIG. 7, it is seen that the slope compensation cannot be decreased unlimitedly, because in order to prevent generation of subharmonic oscillation when the duty cycle exceeds 50%, the slope compensation has to satisfy the following relationship:
                              m          c                >                              m            2                    2                                    (        5        )            
Due to the requirement on the minimum value of mc, when the boost converter operates at a higher duty cycle, this method of decreasing slope compensation cannot achieve a good effect, for example, if the boost converter is at a minimum input voltage and a maximum output voltage, m2 will be the largest, and thus the value of mc will also be the largest.
Besides, it is seen from the equation (1), VSUM=VSENSE+VSLOPE=VC, that if the slope compensation is decreased, the value of VC will be decreased. When the converter operates at a minimum duty cycle and a light load current, the VC value will become very small. Therefore, there is a risk that the boost converter is susceptible to noise interference.
With further reference to FIG. 7, it is seen that even the above method of improving linear transient response by decreasing slope compensation is used, when the boost converter changes from state 1 to state 2, the value of ΔVc is still very large. Therefore, the line transient response is not good yet.